When designing an irrigation reservoir and connecting it to a farm’s irrigation system, it’s wise to start by determining how much water should be stored, whether it’s for primary, supplemental, or emergency irrigation. It’s not always possible to predict these, but an educated guess is better than throwing darts.
Calculate Your Needs
To start your calculations, consider the terrain you have - is it laser leveled or rolling hills? Is your soil very porous and quick to drain or is heavy and full of clay? Consider how many acres need irrigation, and how much (if any) water you expect to draw on an annual basis from existing surface or ground waters. Consider the water needs of your specific crops, the volume of return water you normally collect after irrigating, and the effects of changing local weather patterns (less frequent but stronger rain events, for example), and the volume of stormwater runoff during both growing and dormant seasons. Evaporative loss and seepage should also be taken into consideration.
Do you need to store enough water to take your crops through an entire growing season in case your typical supplies are cut off? Or are you planning on irrigating only occasionally to even out moisture levels when rain is sporadic within a growing season? Try to find a reasonable point where you’ll have an adequate supply to cover most likely scenarios in your area without going overboard. Farmers in the central plains will probably need to store more water than those in the southeast, for example.
If extended droughts are an issue in your area, it’s probably not realistic to store enough water to supply multiple years’ worth of full irrigation. Use your reservoirs to help you plan a full emergency response: in an extreme dry season, consider which crops are you prepared to sacrifice so that you can focus your resources on a few acres of healthy, productive crops. Having a plan in place will help guide irrigation choices throughout the year.
Determine Your Location
It’s preferable to avoid locating your reservoir where you’ll need to sacrifice productive acreage, but other factors need to be weighed in as well:
- Consider how deep the water table sits. Your reservoir must not impinge upon the water table or existing aquifers - it could damage and contaminate valuable water supplies and could compromise the containment of your own reservoir. Plan for a distance of at least 3 feet between the bottom of your reservoir and an existing water table to avoid conflicts.
- How far is the site from your production areas? You’ll need to connect the reservoir to your irrigation equipment, but you’ll also be moving return water back to the reservoir.
- What is the reservoir’s elative elevation compared to your fields? A reservoir that sits at an equal elevation with your fields will require much less power to move water in and out.
- Plan for a level plot. It’s possible to construct a reservoir on slightly sloped land, but the storage volume of the reservoir will suffer.
- How far will the reservoir be from electrical power? Aside from irrigation equipment, pumps and other equipment for maintenance will be in use.
- Will your proposed location affect nearby wildlife by destroying habitat or blocking access to important grazing areas?
- Will you be able to access the reservoir during construction and later for maintenance? Will you install a safety fence and how will that affect access?
- If you’re planning to construct a sequence of settling ponds and reservoirs, ensure your plot is large enough to accommodate the layout and enable either foot or vehicle access on all sides.
Critical Structural Elements
Reservoirs are, fundamentally, a large hole in the ground, with some fancy details tacked on. But as they say, the devil is in the details, and none should be overlooked.
To start with, reservoir sides need to be carefully sloped to minimize erosion along the banks. Erosion will introduce sediment to your reservoir which can damage pumping equipment. Erosion can also threaten the structure of your reservoir by undercutting bank edges. Besides using a slope appropriate for the local soil type, reservoir banks can be reinforced by planting vegetation with dense roots outside the reservoir’s lining.
Not unlike enormous hydroelectric dams, irrigation reservoirs will have one or more inlets, at least one outlet, plus an emergency spillway. The purpose of the emergency spillway is to safely and quickly release excess water in the case of an extreme precipitation event where the water level threatens to overtop surrounding berms or otherwise cause significant damage to the reservoir’s structural integrity.
Inlets and outlets can be constructed with hard materials, planted with vegetation, or virtually anything in between that’s sturdy enough to handle the velocity of water passing by.
Irrigation reservoirs are installed to store water in seasons and locations where water can be difficult to source otherwise, so it makes no sense to keep this precious commodity in a leaky container. Seepage through the bottom of a reservoir reduces the volume of available water and can even erode the structural integrity of your reservoir. It’s common practice to line irrigation reservoirs with impermeable geosynthetics that prevent seepage and also prevent incursion from underground sources into your water. Liners in an irrigation reservoir should be chosen for their durability, resistance to tears and punctures, ability to withstand UV exposure, and ease of installation. BTL Liners has decades of experience in lining large irrigation reservoirs around the world. Don’t hesitate to give us a call to discuss the details of your project with us or ask any questions you may have.
Costs vs Benefits
The cost of building an irrigation reservoir is largely determined by its size and starts with the cost of excavation. Rates will obviously vary across the country, so check pricing when you’ve determined the size, depth, and location of your reservoir(s). Keep in mind that your reservoir will need a permit, and those can add significant costs, depending on your state.
Be prepared to discuss where you plan to source your water, how you’ll treat it (if necessary) and how you’ll apply it to your fields. In some locations, there are incentive programs to promote resiliency and efficient water usage that may help offset the cost of your reservoir or upgrades to your irrigation system. Even with incentive programs, though, a reservoir may not feel like a benefit if you need to install it in valuable cropland, but water is becoming increasingly valuable and the ability to capture and store it as other sources literally dry up may change your perspective. If you sacrifice a few acres of cropland but can store enough water to irrigate 125 acres, that could make all the difference in the world.
Remember that older wisdom about the value of reservoirs may no longer apply. In many parts of the country, precipitation patterns are changing in important ways which make a capture and storage strategy very appealing. When water arrives unpredictably, it’s not the time to start thinking about your options.
